With the rapid development of broadband services, users' demands for access network bandwidth have increased dramatically. Passive Optical Networks (PONs) are currently an important technology for user access. As shown in FIG. 1, in the structure of a related PON system, an Optical Line Terminal (OLT) is connected to an optical splitter through a backbone optical fiber, the optical splitter is connected to multiple Optical Network Units (ONUs) through branch optical fibers, and the OLT communicates with each ONU through one wavelength pair. To implement communication between the OLT and each ONU, the following activation process between the OLT and each ONU needs to be completed first: The OLT opens a quiet window on a downlink wavelength and requests identity information from each ONU (like ONU1, ONU2 . . . ONUn in FIG. 1) through the optical splitter and each ONU reports its own identity information on an uplink wavelength through the optical splitter; after obtaining the identity information of each ONU, the OLT opens the quiet window again on the downlink wavelength and sends a ranging request to each ONU, and likewise, each ONU responds to the ranging request on the uplink wavelength, and the OLT completes ranging. In this way, after the above activation process is completed, the OLT sends data to relevant ONUs according to downlink service conditions and allocates bandwidth to each ONU according to the uplink service status of each ONU or the bandwidth request of each ONU, and each ONU sends uplink data within its own allocated bandwidth.